Among the electronic devices comprising at least a part based on material provided by organic chemistry, organic light emitting diodes (OLEDs) have a prominent position. Since the demonstration of efficient OLEDs by Tang et al. in 1987 (C. W. Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987)), OLEDs developed from promising candidates to high-end commercial displays. An OLED comprises a sequence of thin layers substantially made of organic materials. The layers typically have a thickness in the range of 1 nm to 5 μm. The layers are usually framed either by means of vacuum deposition or from a solution, for example by means of spin coating or jet printing.
OLEDs emit light after the injection of charge carriers in the form of electrons from the cathode and in form of holes from the anode into organic layers arranged in between. The charge carrier injection is effected on the basis of an applied external voltage, the subsequent formation of excitons in a light emitting zone and the radiative recombination of those excitons. At least one of the electrodes is transparent or semitransparent, in the majority of cases in the form of a transparent oxide, such as indium tin oxide (ITO), or a thin metal layer.
Among the matrix compounds used in OLED light emitting layers (LELs) or electron transporting layers (ETLs), important position have the compounds that comprise at least one polar group selected from phosphine oxide and diazole. The reason why such polar group often significantly improves the electron injecting and/or electron transporting properties of the semiconducting material is not yet fully understood. It is believed that the high dipole moment of the polar group plays somehow the positive role. Especially recommended for this use are triaryl phosphine oxides comprising at least one condensed aromatic or heteroaromatic group attached directly to the phosphine oxide group, see e.g. JP 4 876 333 B2. Among diazole groups, especially phenylbenzimidazole groups have been widely utilized for the design of new electron transport matrix compounds, for example TPBI described in U.S. Pat. No. 5,645,948, and some compounds comprising benzimidazolyl structural moieties linked with other structural moieties comprising delocalized pi-electrons in two or more aromatic or heteroaromatic rings nowadays are considered to be industrial standard, e.g. compound LG-201 (for example U.S. Pat. No. 6,878,469)

Electrical doping of charge transporting semiconducting materials for improving their electrical properties, especially conductivity, is known since 1990s, e.g. from U.S. Pat. No. 5,093,698 A. An especially simple method for n-doping in ETLs prepared by the thermal vacuum deposition, which is currently the standard method most frequently used, e.g. in industrial manufacture of displays, is evaporation of a matrix compound from one evaporation source and of a highly electropositive metal from another evaporation source and their co-deposition on a solid substrate. As useful n-dopants in triaryl phosphine oxide matrix compounds, alkali metals and alkaline earth metals were recommended in JP 4 725 056 B2, with caesium as the dopant successfully used in the given examples. Indeed, caesium as the most electropositive metal offers the broadest freedom in the choice of a matrix material, and it is likely the reason why solely caesium was the n-doping metal of choice in the cited document.
For an industrial use, caesium as a dopant has several serious drawbacks. First, it is very reactive, moisture and highly air sensitive material that renders any handling difficult and incurs significant additional costs for mitigating the high safety and fire hazard unavoidably linked with its use. Second, its quite low normal boiling point (678° C.) indicates that it may be highly volatile under high vacuum conditions. Indeed, at pressures below 10−2 Pa used in industrial equipment for vacuum thermal evaporation (VTE), caesium metal evaporates significantly already at slightly elevated temperature. Taking into account that the evaporation temperatures for typical matrix compounds used in organic semiconducting materials at pressures below 10−2 Pa are typically between 150-400° C., avoiding an uncontrolled caesium evaporation, resulting in its undesired deposition contaminating the colder parts of the whole equipment (e.g. the parts that are shielded against heat radiation from the organic matrix evaporation source), is a really challenging task.
Several methods for overcoming these drawbacks and enabling industrial applicability of caesium for n-doping in organic electronic devices have been published. For safe handling, caesium may be supplied in hermetic shells that open just inside the evacuated evaporation source, preferably during heating to the operational temperature. Such technical solution was provided e.g. in WO 2007/065685, however, it does not solve the problem of caesium high volatility.
U.S. Pat. No. 7,507,694 B2 and EP 1 648 042 B1 offer another solution in form of caesium alloys that melt at low temperature and show significantly decreased caesium vapour pressure in comparison with the pure metal. Bismuth alloys of WO2007/109815 that release caesium vapours at pressures of the order 10−4 Pa and temperatures up to about 450° C. represent another alternative. Yet, all these alloys are still highly air and moisture sensitive. Moreover, this solution has further drawback in the fact that the vapour pressure over the alloy changes with the decreasing caesium concentration during the evaporation. That creates new problem of an appropriate deposition rate control, e.g. by programming the temperature of the evaporation source. So far, quality assurance (QA) concerns regarding robustness of such process on an industrial scale hamper a wider application of this technical solution in mass production processes.
A viable alternative to Cs doping represent highly electropositive transition metal complexes like W2(hpp)4 that have ionisation potentials comparably low as caesium and volatilities comparable with volatilities of usual organic matrices. Indeed, these complexes disclosed as electrical dopants first in WO2005/086251 are very efficient for most electron transporting matrices except some hydrocarbon matrices. Despite their high air and moisture sensitivity, these metal complexes provide satisfactory n-doping solution for an industrial use, if supplied in the shells according to WO 2007/065685. Their main disadvantage is their high price caused by relative chemical complexity of comprised ligands and necessity of a multistep synthesis of the final complex, as well as additional costs incurred by necessity of using the protective shells and/or by the QA and logistic issues linked with shell recycling and refilling.
Another alternative represent strong n-dopants created in situ in the doped matrix from relatively stable precursors by an additional energy supplied e.g. in form of ultraviolet (UV) or visible light of an appropriate wavelength. Appropriate compounds for this solution were provided e.g. in WO2007/107306 A1. Nevertheless, state-of-art industrial evaporation sources require materials with very high thermal stability, allowing their heating to the operational temperature of the evaporation source without any decomposition during the whole operating cycle (e.g., for a week at 300° C.) of the source loaded with the material to be evaporated. Providing organic n-dopants or n-dopant precursors with such long-term thermal stability is a real technical challenge so far. Moreover, the complicated arrangement of the production equipment that must ensure a defined and reproducible additional energy supply for achieving reproducibly the desired doping level (through the in situ activation of the dopant precursor deposited in the matrix) represents an additional technical challenge and a potential source of additional CA issues in mass production.
Yook et al (Advanced Functional Materials 2010, 20, 1797-1802) successfully used caesium azide in laboratory as an air-stable Cs precursor. This compound is known to decompose under heating above 300° C. to caesium metal and elemental nitrogen. This process is, however, hardly applicable in contemporary industrial VTE sources, due to difficult control of such heterogeneous decomposition reaction in a larger scale. Moreover, release of nitrogen gas as a by-product in this reaction brings a high risk that, especially at higher deposition rates desired in the mass production, the expanding gas will expel solid caesium azide particles from the evaporation source, causing thus high defect counts in the deposited layers of doped semiconducting materials.
Another alternative approach for electrical n-doping in electron transporting matrices is doping with metal salts or metal complexes. The most frequently used example of such dopant is lithium 8-hydroxy-quinolinolate (LiQ). It is especially advantageous in matrices comprising a phosphine oxide group, see e.g. WO 2012/173370 A2. The main disadvantage of metal salt dopants is that they improve basically only electron injection to the adjacent layers and do not increase the conductivity of doped layers. Their utilization for decreasing the operational voltage in electronic devices is thus limited on quite thin electron injecting or electron transporting layers and does hardly allow e.g. an optical cavity tuning by using ETLs thicker than approximately 25 nm, what is well possible with redox-doped ETLs having high conductivity. Furthermore, metal salts typically fail as electrical dopants in cases wherein creation of new charge carriers in the doped layer is crucial, e.g. in charge generating layers (CGL, called also p-n junctions) that are necessary for the function of tandem OLEDs.
For the above reasons, and especially for electrical doping in ETLs thicker than approximately 30 nm, the current technical practice prefers lithium as an industrial redox n-dopant (see e.g. U.S. Pat. No. 6,013,384 B2). This metal is relatively cheap and differs from other alkali metals by its somewhat lower reactivity and, especially, by its significantly lower volatility (normal boiling point about 1340° C.), allowing its evaporation in the VTE equipment at temperatures between 350-550° C.
Nevertheless, quite in accordance with its high n-doping power allowing Li to dope majority of usual types of electron transporting matrices, this metal possesses also a high degree of reactivity. It reacts under ambient temperature even with dry nitrogen and for its use in a highly reproducible manufacturing process complying with contemporary industrial QA standards, it must be stored and handled exclusively under high purity noble gases. Moreover, if Li is co-evaporated with matrix compounds that have evaporation temperatures in the range 150-300° C., its significantly higher evaporation temperature in comparison with the matrix evaporation temperature already causes cross-contamination problems in the VTE equipment.
Many documents suggest as alternative n-dopants almost any known metallic element including weakly reductive and highly volatile Zn, Cd, Hg, weakly reductive Al, Ga, In, Tl, Bi, Sn, Pb, Fe, Co, Ni, or even noble metals like Ru, Rh, Ir and/or refractory metals with highest known boiling points like Mo, W, Nb, Zr (see e.g. JP 2009/076508 or WO 2009/106068). Unfortunately, not only in these two documents cited here as examples but throughout the scientific and patent literature overall, there is in fact lack of any evidence that some of these suggestions have ever been experimentally tested.
To be more specific, even WO 2009/106068 that does not merely mention all imaginable dopants but really strives to claim all the named metalloid elements as n-dopants in organic electronic devices due their alleged applicability through a high-temperature decomposition of a gaseous precursor compound in a heated nozzle, does not bring any single numeric value documenting the physical parameters of allegedly prepared doped materials and/or technical performance of allegedly prepared devices.
On the other hand, US2005/0042548 published before the date of priority of WO 2009/106068 teaches in paragraph 0069 (see namely the last two lines of the left column and first three lines of the right column on page 7) that iron pentacarbonyl can be used for n-doping in organic ETMs if the compound is activated by UV radiation which splits off a carbon monoxide ligand. The coordinatively unsaturated iron compound then reacts with the matrix, what results in the observed doping effects. In the light of this previous art showing that the metal carbonyls that were used in the alleged working example of WO 2009/106068 are known n-dopants in organic matrices if activated by supply of additional energy, it seems quite likely that if the applicants of WO2009/106068 really obtained with their jet of iron pentacarbonyl flowing through a ceramic nozzle electrically heated to a white glow (see the last paragraph of the German text on page 12 of the cited PCT application) any doping effect in the target bathocuproin layer, this effect was caused rather by the same coordinatively unsaturated iron carbonyl complex as produced by UV irradiation in US2005/0042548, than with elemental iron as they suggest. This suspicion is further supported by the fourth paragraph on page 13 of the cited PCT application which teaches that the same result can be obtained with a cold nozzle, if the stream of iron pentacarbonyl is irradiated with an infrared laser having the wavelength fitting with the absorption frequency of the CO groups in the iron pentacarbonyl complex. Here, it is even more likely that the laser activation resulted not in naked metal atoms or clusters of metal atoms but in a reactive, coordinately unsaturated iron complex still bearing some carbonyl ligands, analogously to the reactive complex formed by activation with the UV light.
Despite metals with strongly negative standard redox potentials like alkali earth metals or lanthanides are recited as alternative n-dopants besides alkali metals basically in each document dealing with redox n-doping, the record of the proven n-doping with any metal different from alkali metals is very scarce.
Magnesium is in comparison with alkaline metals much less reactive. It reacts even with liquid water at the ordinary temperature very slowly and in air it keeps its metallic luster and does not gain weight for months. It may be thus considered as practically air-stable. Moreover, it has low normal boiling point (about 1100° C.), very promising for its VTE processing in an optimum temperature range for co-evaporation with organic matrices.
On the other hand, the authors of the present application confirmed in a screening done with dozens of state-of-art ETMs that Mg does not possess a sufficient doping strength for common ETMs which are free of strongly polar groups like phosphine oxide group. The only favourable result has been achieved in OLEDs comprising thin electron injection layers consisting of a specific kind of triaryl phosphine oxide matrix (comprising a special tris-pyridyl unit designed for chelating m s), doped with magnesium, as shown in EP 2 452 946 A1. Despite the structural specifity and very favourable (in terms of its LUMO level which is quite deep under the vacuum level in the absolute energy scale) dopability of the exemplary matrix tested with magnesium in EP 2 452 946 A1, the positive results achieved with this n-doped semiconducting material encouraged further research focused on n-doping with substantially air stable metals.
It is an object of the invention to overcome the drawbacks of the prior art and to provide effectively n-doped semiconducting materials, preferably utilizing substantially air stable metals as n-dopants, especially in ETMs having their lowest unoccupied molecular orbital (LUMO) energy levels closer to vacuum level than the ETMs which have electrochemical redox potentials (that are in a simple linear relationship with the LUMO levels and are much easier measurable than LUMO levels themselves) with more negative values than about −2.25 V against ferrocenium/ferrocene reference.
It is a further object of the invention to provide alternative metallic elements which are substantially air stable and can be successfully embedded (preferably by standard VTE processes and using contemporary evaporation sources) in electrically doped semiconducting materials for use in electronic devices.
A third object of the invention is to provide a process for manufacturing the semiconducting material utilizing substantially air stable metals as n-dopants.
A fourth object of the invention is to provide devices with better characteristics, especially with low voltage and, more specifically, OLEDs with low voltage and high efficiency.
A fifth object of the invention is to provide new matrix compounds applicable in semiconducting materials according to the invention.